Animals Act, No. 29 of 1958

The Animal Act aims to regulate the slaughtering of animals, removal of animals from one administrative district to another. The Act also makes provision for branding of animals and for the issue of vouchers for the animals branded. The Act regulates the sale and transfer of such animals.
It regulates the trespassing of the animals and also provides for the measures for the improvement of the breed of animals. 

Animals Act Regulations Amendment 2009 No. 10
http://www.daph.gov.lk/web/images/content_image/vra/acts/Animals_Act_Regulations_2009.pdf

Section 28 of the Act states that no person shall have in his possession any male animal over the age of one year and three months unless such animal
a) has been castrated within three months after attaining the age of one year; or
b) has been certified by a Veterinary Surgeon or an authorized officer to be fit for breeding.
Section 33 states that where any Veterinary Surgeon or authorized officer is of the opinion that any male animal over the age of one year is unfit for breeding, that Veterinary Surgeon or authorized officer may direct the owner of, or the person in possession of, that animal by notice in writing addressed to such owner or person, to castrate that animal, and such owner or person shall carry out such direction.
These provisions, can be used to prevent any transgenic, animal felling within the definition from breeding with native animals by ordering such males to be castrated.